This invention relates to a light source for generating ultrashort optical pulses having a narrow spectral band width, and a voltage detection apparatus using the light source in which the optical pulses are utilized as sampling gates for measuring an ultrashort voltage variation.
In recent years, rapid progress has been made in the development of an electrooptic sampling technique for detecting repetitive ultrafast electrical transients at a higher time (temporal) resolution ranging from pico-seconds to femto-seconds, which can not be detected by an electrically-measuring technique such as a sampling oscilloscope or the like. This electrooptic sampling technique is an electric field sensitive technique which utilizes ultrashort optical pulses as sampling gates (optical probe) to detect the ultrafast electrical transients in electronic devices such as field-effect transistors, photodetectors and high-speed integrated circuits with high temporal and spatial resolution.
One device utilizing the above technique is known as an electrooptic sampling device in which optical pulses are used as sampling gates or probes to measure time-variation of an electrical signal flowing in an electrical element using an electrooptic (E-O) effect of an electrooptic (E-O) medium (crystal).
In this device, the E-O medium having a reflection surface is disposed approximately to the electrical element to be measured such that the reflection surface confronts the electrical element, and optical pulses which are irradiated to the E-O medium as an optical probe pulse, are passed through the E-O medium and reflected from the reflection surface to a photodetector. When the electrical signal having a voltage-waveform is applied to the electrical element, an electric field in a space which surrounds the electrical element is changed in accordance with the voltage-waveform of the electrical signal. In this case, the E-O medium disposed in the space is electrooptically effected by the change of the electric field and thus the optical pulses which have been passed through and reflected from the E-O medium are optically changed in polarization in accordance with the change of the electric field (or the voltage-waveform of the electrical signal). Each of the optical pulses reflected from the E-O medium is inputted to an analyzer to convert change of polarization into change of intensity and then the change of intensity is detected by the photodetector, and light intensity thereof is integrated for a period corresponding to a pulse width to thereby output an integrated value every optical pulse. These integrated values for the optical pulses are two-dimensionally displayed in a time axis on a display, and a profile of the displayed integrated values with respect to the time axis represents a measured voltage-waveform of the electrical signal. This device is disclosed by Janis A. Valdmanis and G. Mourou, "Subpicosecond Electrooptic Sampling: Principles and Applications", IEEE Journal of Quantum Electronics, Vol. QE-22, No. 1, January 1986, pages 69-78).
Another type of device of measuring an electrical signal in non-contact fashion using ultrashort optical pulses is disclosed in Japanese Unexamined Published Patent Publications Nos. 63-300970 and 64-46659.
In these techniques, the time or temporal resolution of the device is determined by the convolution time of an optical sampling (probe) pulse and a traveling electrical signal (variable electric field) in an electrooptic medium. If the pulse width of the optical sampling pulse is remarkably narrower in comparison with the voltage-waveform of the electrical signal, the time or temporal resolution is improved. Further, if a repetitive frequency of the optical pulses at which the voltage-waveform of the electrical signal is detected by a train of the optical sampling (probe) pulses is more increased, a light amount of the optical sampling pulses per second to be used for a sampling operation is more increased. Therefore, the higher repetitive frequency of the optical pulses enables a signal-to-noise (S/N) ratio to be more improved and enables a measuring time to be shorter. Accordingly, there has been required optical pulses having a shorter pulse width (duration) and a higher repetitive frequency in these techniques.
In order to perform a measuring operation of an electrical signal with high time resolution, a dye laser for generating optical pulses whose pulse width (duration) ranges from picoseconds to femtoseconds is advantageous, but results in a large-size system. In view of this disadvantage of the dye laser, a small-size semiconductor laser has been proposed to be used as a pulse light source as in SPIE Vol. 1155, pages 499-510. At present, a practically usable pulse width (duration) of a ultrashort optical pulse emitted by semiconductor lasers ranges from picoseconds to 200 picoseconds.
On the other hand, a wavelength of the emitted optical pulse varies in accordance with a type of semiconductor laser, and is usually in the range of from 670 nm to 1.5 .mu.m. If a second harmonic wave of the optical pulse emitted from the semiconductor laser is generated, then an optical pulse whose wavelength is up to 340 nm can be obtained. The repetition frequency of such an optical pulse is generally in the range of from 0.1 to 200 MHz, depending on an application field to which the optical pulse is used. Technically, it is possible to produce repetitive optical pulses in an ultrahigh frequency range of several hundred Hz to several GHz.
In the electrooptic sampling technique as described above, a Fabry-Perot type semiconductor laser in which a pair of reflection plates are disposed so as to be confronted with each other to develop an oscillation therebetween is used as a light source for generating optical pulses. When the semiconductor laser is energized for a pulse oscillation, the semiconductor laser is oscillated in multiple modes and an optical pulse generated by the semiconductor laser has a broader spectral waveform (in wavelength) as shown in FIG. 1(B), unlike a narrow spectral waveform of CW oscillation as shown in FIG. 1(A).
Upon application of such an optical pulse having the broader spectral waveform as shown in FIG. 1(B) to an optical modulator such as an electrooptic crystal, a voltage (halfwave voltage V.pi. or V.lambda./2) which is applied to the optical modulator to make a phase difference of between perpendicularly-polarized components of the optical pulse at an exit of the electrooptic crystal has various values. The voltage has no constant value because the optical pulse has a broad spectral band width and the halfwave voltage is different among lights of these wavelengths in the band. More specifically, in the case where an electrooptic crystal is used in an optical modulator, the halfwave voltage V.pi. of the optical modulator must be changed in accordance with the wavelength of light applied thereto. For example, a higher halfwave voltage must be used in a longer wavelength region of the spectral band and a lower halfwave voltage must be used in a shorter wavelength region of the spectral band.
As described above, the half-wavelength voltage V.pi. is changed in accordance with the wavelength of the incident light. The relationship between the applied voltage to the electrooptic crystal and the intensity of an output light of the light modulator also and varied in accordance with the wavelength. For example, as shown in FIG. 2, if the semiconductor laser oscillates in multiple modes and the produced light has a wide spectral band ranging from .pi.1 to .pi.2 with a central wavelength .pi.0 (.pi.1&lt;.pi.0&lt;.pi.2), then an optical characteristic of the optical modulator (that is, a relationship between an applied voltage and a relative output light intensity) is changed in accordance with the wavelength.
On the other hand, in the optical modulator as described above, an optical condition (state) which is obtained by applying a voltage to the electrooptic crystal of the optical modulator is also obtained by adjusting an optical system such as a phase compensator (not shown) which is further equipped to the optical modulator. Thus it is a practical manner to establish an optical condition corresponding to V.pi./2 as an operating point by the above optical adjustment in order to maximize a change in the output light intensity in response to a change in the applied voltage (that is, to use a steeper portion of the S-curved optical characteristic as shown in FIG. 2). However, even if the operating point is set at V.pi./2 (at a point A in FIG. 2) of the wavelength .lambda.Q, the operating point is shifted to a point B for the wavelength .lambda.1 or to a point C for the wavelength .lambda.2, and it does not remain at V.pi./2 for the wavelength .lambda.1 or .lambda.2, i.e., deviates from points B', C'. Furthermore, in response to the change in the applied voltage, the output light intensity varies at different rates with the wavelengths .lambda.1, .lambda.0, .lambda.2. Accordingly, the electric signal applied to the light modulator is modulated in a different modulating manner every wavelength.
The same problem occurs when the output light of a semiconductor laser has a mode hopping or a wavelength shift due to a temperature change.
As shown in FIG. 1(B), an optical pulse emitted from the semiconductor laser has a Gaussian type of spectral waveform having the highest peak at the central portion in a spectral band on the time-average, but the spectral waveform of the optical pulse is changed instantaneously. For example, the wavelength of the highest peak is successively shifted to any position within the spectral band. This shift or change of the spectral waveform at instantaneous time is called "mode hopping". Further, the spectral waveform of the optical pulse is wholly shifted upwardly or downwardly with respect to the wavelength in accordance with the change of temperature. For example, the spectral wavelength is wholly shifted to a lower wavelength side as the temperature is decreased, while the spectral wavelength is wholly shifted to a higher wavelength side as the temperature is increased. This shift of the spectral wavelength is the wavelength shift due to temperature change.
Accordingly, even if an electric signal of the same waveform is applied, the change in the intensity of the output light of the optical modulator is different due to the mode hopping and/or the wavelength shift due to the temperature change. Since the electrooptic sampling technique measures the change of the applied voltage on the basis of the change in the output light intensity, the mode hopping or wavelength shift of the semiconductor laser results in noise, so that the voltage is prevented from being accurately measured and the accuracy for measurement is degraded due to the mode hopping and the wavelength shift. In case of using an optical pulse having a broad spectral band width, the difference in a propagating speed between the lights having different wavelengths within a dispersion medium such as an electrooptic crystal is expanded by the dispersive medium. As a result, the pulse duration of the optical pulse having these lights is further increased, and the time resolution upon measurement is lowered.